1, PSD, AI,CDA are the best formats. PDF, TIFF& JPG are also
acceptable.

2, Any format artwork must be 300dpi or above.

How does the flip pictures produced?

From there the interlaced image can be printed directly to the back
(smooth side) of the lens or it can be printed to a substrate
(ideally a synthetic paper) and laminated to the lens. When
printing to the backside of the lens, the critical registration of
the fine "slices" of interlaced images must be absolutely correct
during the lithographic or screen printing process or "ghosting"
and poor imagery might result. Ghosting also occurs on choosing the
wrong set of images for flip.

FAQ

Q: What is LPI?

A: LPI means the number of lines installed in one inch (2.54 cm)
wide

LPI is bigger, the picture is more clear and alive, 3D effect is
better